The only advantage to the SDK will be fullly documented OS features, not all of which may be known right now (though I'm tempted to think that current devs have pretty much managed to suss it out pretty nicely). Apart from that, and if anything, the SDK will have more restrictions on what developers can do, not less. Apple has designed the SDK to be relatively limited in what you can do with the OS in order to prevent malware from being developed and mucking up people's units.

Furthermore, the quality of the apps will depend entirely on the developer. It's likely that, due to the digital signature requirement of apps developed with the SDK (which will cost money) you're probably only going to see more professional companies jump on board, and possibly some hardcore hobbyists (depending on the cost of signing). Given that, and notwithstanding current "killer apps," the overall average quality may be higher than the average app right now. But we'll see. It's unlikely anything useful will come out for another couple of months yet -- devs have to get it into their hot little hands first (next month) and then learn the lay of the land and write something for it.